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Positron Annihilation Spectroscopy and Small Angle Neutron Scattering Characterization of Nanostructural Features in High-Nickel Model Reactor Pressure Vessel Steels

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Positron Annihilation Spectroscopy and Small Angle Neutron Scattering Characterization of Nanostructural Features in High-Nickel Model Reactor Pressure Vessel Steels Journal of Nuclear Materials 351 (2006) 197–208 www.elsevier.com/locate/jnucmat Positron annihilation spectroscopy and small angle neutron scattering characterization of nanostructural features in high-nickel model reactor pressure vessel steels Stephen C. Glade a, Brian D. Wirth a,*, G. Robert Odette b, P. Asoka-Kumar c a Nuclear Engineering Department, University of California, Berkeley, CA 94720-1730, USA b University of California, Santa Barbara, CA, USA c Lawrence Livermore National Laboratory, Livermore, CA, USA Abstract Irradiation embrittlement in nuclear reactor pressure vessel steels results from the hardening by a high number density of nanometer scale features. In steels with more than 0.10% Cu, the dominant features are often Cu-rich precipitates typically alloyed with Mn, Ni and Si. At low-Cu and low-to-intermediate Ni levels, so-called matrix hardening features are believed to be vacancy-solute cluster complexes, or their remnants. However, Mn–Ni–Si rich precipitates, with Mn plus Ni contents greater than Cu, can form at high alloy Ni contents and are promoted at irradiation temperatures lower than the nominal 290 °C. Even at very low-Cu levels, late blooming Mn–Ni–Si rich precipitates are a significant concern due to their potential to form large volume fractions of hardening features. Positron annihilation spectroscopy (PAS) and small angle neutron scattering neutron (SANS) measurements were used to characterize the fine-scale microstructure in split- melt A533B steels with varying Ni and Cu contents, irradiated at selected conditions from 270 to 310 °C between 0.04 and 1.6 · 1023 nmÀ2. The objective was to assess the character, composition and magnetic properties of Cu-rich precipitates, as well as to gain insight on the matrix features. The results suggest that the irradiated very low-Cu and inter- mediate Ni steel contains small vacancy-Mn–Ni–Si cluster complexes, but not large, well-formed and highly enriched Mn–Ni–Si phases. The hardening features in steels containing 0.2% and 0.4% Cu, and 0.8% and 1.6% Ni are consistent with well-formed, non-magnetic Cu–Ni–Mn precipitates. The precipitate number densities and volume fractions increase, while their sizes decrease, with increasing Ni and decreasing irradiation temperature. The precipitates evolve with fluence in stages of nucleation, growth and limited coarsening. Ó 2006 Elsevier B.V. All rights reserved. 1. Introduction Si–Mo–Ni low-alloy bainitic steels. RPVs operate at around 290 °C accumulating fast neutron Western reactor pressure vessel (RPV) steels are fluences from about 1 to 10 · 1023 nmÀ2 over a fabricated from quenched and tempered C–Mn– 40–60 year service life, corresponding to a maxi- mum damage dose less than 0.15 displacements * Corresponding author. Tel.: +1 510 642 5341; fax: +1 510 643 per atom (dpa) [1–4]. However, even this relatively 9685. low dose is sufficient to produce severe embrittle- E-mail address: [email protected] (B.D. Wirth). ment in some cases, characterized by upward shifts 0022-3115/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnucmat.2006.02.012 198 S.C. Glade et al. / Journal of Nuclear Materials 351 (2006) 197–208 in the transition temperature (DT) between the more and fluence. In general, the results derived from brittle cleavage and more ductile microvoid coales- the different techniques are in good agreement, cence fracture regimes. although some questions remain regarding the com- Embrittlement is primarily the result of irradia- position of the precipitates, particularly with respect tion hardening, reflected in yield strength increases, to their possible Fe content [13]. Specifically, some with corresponding DT values of 300 °C or more. atom probe data has been interpreted to suggest Embrittlement is controlled by a complex combina- the precipitates contain up to 50% or more Fe, tion of variables, including the neutron flux, fluence which is inconsistent with the results of other tech- and energy spectrum, the irradiation temperature niques. It is believed that some Cu catalyzes MNP (irradiation variables), and the alloy’s starting formation, but they can also form even in steels with microstructure and composition (material variables) little or no Cu at sufficiently high fluence, depending [1–6]. Important compositional variables include Cu on the irradiation temperature and alloy Ni and Mn (0.02–0.5%), Ni (0.2–2%) and Mn (0.3–2%), while P content. Once nucleated, MNPs grow rapidly to (0.005–0.040%) and Si (0.2–0.7%) generally play large volume fractions, with a correspondingly high secondary roles (note all compositions in this paper level of hardening and embrittlement [1,2]. How- are given in weight percent unless otherwise noted). ever, a complete understanding of the necessary The principal source of irradiation hardening is the conditions for MNP formation and evolution as a formation of high number densities of nanometer- function of irradiation and material variables does sized irradiation-induced precipitates and so-called not yet exist. matrix features, which impede the motion of dislo- In this paper, we present the results of a positron cations [1–6]. In Cu-bearing steels, irradiation hard- annihilation spectroscopy (PAS) and small angle ening is primarily associated with the accelerated neutron scattering (SANS) study to characterize formation of nanometer-sized Cu-rich precipitates the nanometer precipitates and matrix features (CRPs) due to radiation-enhanced solute diffusion. formed in a series of model RPV steels with con- The CRPs are enriched in Ni, Mn and Si. Mn–Ni trolled variations in Cu and Ni contents following rich precipitates (MNPs) containing a range of selected neutron irradiation at a flux of 3.2 ± Cu, as dictated by the alloy content of this element, 0.6 · 1015 nmÀ2 sÀ1 to fluences from 0.04 to 1.6 · form at high alloy Mn and Ni levels, and are pro- 1023 nmÀ2 over the temperature range of 270– moted by lower irradiation temperatures (<290 °C) 310 °C. The results, which extend our previous mea- and Cu contents [2,7]. Indeed, nearly pure well- surements on model alloys, provide quantitative formed Mn–Ni–Si phases develop even in very information on the composition, sizes, number low-Cu steels at sufficiently high fluence and Ni densities, volume fractions and magnetic properties contents (possibly > 1%), although the tempera- of the Cu–Ni–Mn precipitates (CRPs), as well as ture–alloy composition MNP phase boundaries further qualitative insight on the matrix features. have not been experimentally established. Smaller, so-called matrix features, believed to be vacancy- 2. Experimental methods solute (e.g., Mn, Ni and Si) cluster complexes, or their solute remnants, also contribute to hardening, 2.1. Materials and irradiations even in alloys with low or no copper [3,8]. The matrix features may be precursors to MNP Commercial model (CM) A533B-type split-melt formation. bainitic steels with controlled variations in Cu and The CRPs [9] and MNPs [1], first predicted by Ni content, as shown in Table 1, were examined in Odette, have been identified and characterized by this study. The matrix consisted of a low-Cu, inter- a combination of experimental techniques, includ- mediate Ni steel (CM3) and intermediate (CM16 ing small angle neutron scattering (SANS) [2,10– and CM17) and high (CM19 and CM20) Cu alloys 13], atom probe field ion microscopy (APFIM) with both intermediate (CM16 and CM19) and high [13–19], combined electrical resistance Seebeck coef- (CM17 and CM20) Ni contents. These CM-series ficient measurements (RSC) [19–21], and recently, steels had typical prior austenite grain sizes of positron annihilation spectroscopy (PAS) [22–26]. 50 lm and tempered bainite microstructures. The The characteristics of the CRPs and MNPs have heat treatment schedule was as follows: austenitize been studied as a function of steel composition, heat at 900 °C for 0.5 h, salt bath quench to 450 °C treatment, irradiation temperature, neutron flux and hold for 600 s, temper at 660 °C for 4 h, stress S.C. Glade et al. / Journal of Nuclear Materials 351 (2006) 197–208 199 Table 1 Measured compositions of the model steels used in this study Steel Cu (wt%) Ni (wt%) Mn (wt%) Mo (wt%) C (wt%) P (wt%) Si (wt%) S (wt%) CM3 0.02 (low) 0.85 (intermediate) 1.60 0.49 0.13 0.006 0.16 0.000 CM16 0.22 (intermediate) 0.82 (intermediate) 1.58 0.51 0.16 0.004 0.25 0.000 CM17 0.22 (intermediate) 1.59 (high) 1.54 0.50 0.16 0.004 0.25 0.000 CM19 0.42 (high) 0.85 (intermediate) 1.63 0.51 0.16 0.005 0.16 0.003 CM20 0.43 (high) 1.69 (high) 1.63 0.50 0.16 0.006 0.16 0.003 All values are given in weight percent. relieve at 607 °C for 24 h, cool at 8 °C/h to 300 °C. (or lower) positron affinity than the matrix they Air cooling followed all the steps in the heat treat- are embedded in, also leading to positron localiza- ment sequence described above. tion in these regions. In this study of model Approximately 1 · 1 · 0.2 cm coupons were A533B-type steels, the positron affinities of the neutron irradiated in the University of California, key elements are: À3.72 eV Mn, À3.84 eV Fe, Santa Barbara (UCSB) irradiation variable (IVAR) À4.46 eV Ni and À4.81 eV Cu [29]. As shown by experiment at the University of Michigan Ford Nagai et al. [22], PAS is an especially well suited Nuclear Reactor. The neutron fluxes, fluences and technique for studying CRPs, since Cu clusters as irradiation temperatures are shown in Table 2.
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